1. Field of the Invention
The present invention relates to an optical pickup which reads recorded information from an optical disk, and, more particularly, to an optical pickup which is compatible to both a DVD (Digital Video Disk) and a CD (Compact Disk).
2. Description of Background Information
DVDs are optical disks which can record a vast amount of digital information, and are designed to be 12 cm in diameter, the same as the diameter of CDs, and be able to record digital information, such as moving pictures or computer data, at a recording density of six to eight times that of CDs. To achieve such high-density recording, various tools are designed for DVDs. For SDs (Super-density Disks) which are one type of DVDs, for instance, in order to improve the recording capacity, the wavelength of the laser light source to be used is set to 650 nm or 635 nm, shorter than the wavelength of 780 nm for CDs, the number of apertures NA of the objective lens is set to 0.6, greater than that (0.45) for CDs, and a data compression algorithm as defined in MPEG 2 is used. Accordingly, high density recording of approximately 5 gigabytes (about eight times that for CDs) on one side of each disk.
Because DVDs accomplish significantly higher density recording than CDs as mentioned above, the spot size of the laser beam to read pit information should be set considerably smaller than that for CDs. The spot size of the laser beam is proportional to the wavelength .lambda. of the laser in use and is inversely proportional to the numerical aperture NA of the objective lens. For DVDs, the spot size of the laser beam is decreased by using a laser light source with a short wavelength and an objective lens with a large numerical aperture NA.
It is desirable that DVDs and CDs be optical disks of the same recording type and CDs be playable by a DVD player. When the wavelength .lambda. of the laser is shortened and the numerical aperture NA of the objective lens is increased for DVDs, however, wavefront aberration (mainly coma aberration) occurs even when the disk is slightly inclined. This reduces the margin for the angle of shifting of the disk surface from the perpendicular position with respect to the optical axis of the optical pickup, i.e., the so-called tilt angle. When CDs have a thickness greater than DVDs (e.g, when DVDs have a thickness of 0.6 mm whereas CDs have a thickness of 1.2 mm), wavefront aberration (mainly spherical aberration) occurs at the time CDs are played. This aberration widens the spot size of the laser beam. Therefore, information cannot be read from CDs by directly using the optical pickup for DVDs.
Some schemes have been proposed to overcome the above problem. The schemes include a method of using two objective lenses respectively for DVDs and CDs and switching the lenses to the desired one depending on the types of the target disk, a method of inserting a compensation lens in the collimator portion to correct the disk-based aberration, and a method of utilizing a hologram-used 2-focus lens as the objective lens.
The use of two objective lenses and the use of a compensation lens complicate the mechanism to require larger space, which involve a problem that the whole optical pickup cannot be made compact. The method which uses holograms utilizes diffraction and interference and involves a multibeam, so that this method suffers low light using efficiency and is apt to be influenced by the multibeam oriented interference.